Engineering Fracture Mecha PERGAMON Engineering Fracture Mechanics 69(2002)533-553 www.elsevier.com/locate/engfracmech The past, present, and future of fracture mechanics B. Cotterell Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore The science of fracture mechanics was born and came to maturity in the 20th century. Its literature is now vast. Perhaps the most successful application of fracture mechanics is to fatigue. However, this short paper is limited to a core topic of fracture, the initiation and propagation of fracture under monotonic loading at low strain rates. As the author was invited to present this paper on the occasion of his 65th birthday, it is a somewhat personal view of the development and future of fracture mechanics, but it is hoped that it will interest many, especially the young researchers at the beginning of their careers. g 2002 Elsevier Science Ltd. All rights reserved 1. Introduction The start of new millennium is an appropriate time to stand back and look at the development of fracture mechanics. Fracture will always have a wide-ranging importance to man. An attempt to illustrate this importance is shown in Fig. I. The first illustration is a stone hand-axe dating from the Palaeolithic era However, man and his hominid ancestors had been fashioning stone for more than a million years when it was made. Once man built large expensive structures, he had to ensure that they did not fracture and collapse. Fracture mechanics is basically about scaling. Leonardo da Vinci(1452-1519) was the first to record an understanding the scaling of fracture and the second illustration in Fig. I comes from one of his notebooks [l] and illustrates his strength tests on iron wires. Galileo Galilei(1638)writing in his"Dialogues Concerning Two New Sciences"[2] was the first to give the correct scaling laws for bars under tension and nding. His illustration for a discussion of the fracture of beams is shown in Fig. 1. Size effect is very important in fracture and Galileo saw that this effect placed a limit on the size of structures, both man-made and natural, which makes it impossible to build'ships, palaces, or temples of enormous size in such a way lat all their oars, yards, beams, iron bolts, etc. will hold together, nor can nature produce trees of ex traordinary size because their branches would break down under their own weight Iron, and from the 1860s, steel saw increasing structural use in the 19th century and fracture was a problem. David Kirkaldy opened his Testing and Experimental Works in 1865 and his testing mark is illustrated in Fig. 1. He published a comprehensive account of his experiments [3] and discussed some of the fracture problems with steel, the new structural material. However, the most prophetic pronouncement about the brittle fracture problems with steel that were to come, occurred in 1861 in a leading article of E-mail address: brian-c@imre. org. sg(B Cotterell 00137944/02/S- see front matter 2002 Elsevier Science Ltd. All rights reserved PI:S0013-7944(01)001011
The past, present, and future of fracture mechanics B. Cotterell Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore Abstract The science of fracture mechanics was born and came to maturity in the 20th century. Its literature is now vast. Perhaps the most successful application of fracture mechanics is to fatigue. However, this short paper is limited to a core topic of fracture, the initiation and propagation of fracture under monotonic loading at low strain rates. As the author was invited to present this paper on the occasion of his 65th birthday, it is a somewhat personal view of the development and future of fracture mechanics, but it is hoped that it will interest many, especially the young researchers at the beginning of their careers. 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The start of new millennium is an appropriate time to stand back and look at the development of fracture mechanics. Fracture will always have a wide-ranging importance to man. An attempt to illustrate this importance is shown in Fig. 1. The first illustration is a stone hand-axe dating from the Palaeolithic era. However, man and his hominid ancestors had been fashioning stone for more than a million years when it was made. Once man built large expensive structures, he had to ensure that they did not fracture and collapse. Fracture mechanics is basically about scaling. Leonardo da Vinci (1452–1519) was the first to record an understanding the scaling of fracture and the second illustration in Fig. 1 comes from one of his notebooks [1] and illustrates his strength tests on iron wires. Galileo Galilei (1638) writing in his ‘‘Dialogues Concerning Two New Sciences’’ [2] was the first to give the correct scaling laws for bars under tension and bending. His illustration for a discussion of the fracture of beams is shown in Fig. 1. Size effect is very important in fracture and Galileo saw that this effect placed a limit on the size of structures, both man-made and natural, which makes it impossible to build ‘ships, palaces, or temples of enormous size in such a way that all their oars, yards, beams, iron bolts, etc. will hold together; nor can nature produce trees of extraordinary size because their branches would break down under their own weight’. Iron, and from the 1860s, steel saw increasing structural use in the 19th century and fracture was a problem. David Kirkaldy opened his Testing and Experimental Works in 1865 and his testing mark is illustrated in Fig. 1. He published a comprehensive account of his experiments [3] and discussed some of the fracture problems with steel, the new structural material. However, the most prophetic pronouncement about the brittle fracture problems with steel that were to come, occurred in 1861 in a leading article of Engineering Fracture Mechanics 69 (2002) 533–553 www.elsevier.com/locate/engfracmech E-mail address: brian-c@imre.org.sg (B. Cotterell). 0013-7944/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 7 9 4 4 ( 0 1 ) 0 0 1 0 1 - 1
B. Cotterell Engineering Fracture Mechanics 69(2002)533-553 ? 500360130908040 1060453010 Years Past Future Fig. 1. The fracture perspective The Engineer"that is worth quoting: "Effects of percussion and frost upon iron. We need hardly say that this is one of the most important subjects that engineers of the present day are called upon to investigate The lives of many persons, and the property of many more, will be saved if the truth of the matter be discovered--lost if it be not"[4] Apart from the pioneering work, the understanding of fracture waited until the last century of the second Millennium. Even then it took until well into the second half of this century until a rational approach was used by engineers and taught in engineering schools. The author vividly remembers being taught the five theories of strength as an undergraduate in the early 1950s without any real explanation. A major spur to the development of fracture theory from 1940 onwards was the brittle fracture of large welded structures such as bridges, ships and oil storage containers. This period is illustrated in Fig. 1 by a photograph of the Schenectady that broke in two while lying in the fitting-out dock in January 1943. One of the most successful applications of linear elastic fracture mechanics(LEFM)is to the reliability assessment of aircraft. a picture of the ill-fated Comet is shown in Fig. 1. A fast fracture initiated from a fatigue crack that grew from an astro-navigation window and caused the loss of two aircraft in 1953 and 1954. One of the first applications of LEFM was an explanation of this fast fracture [5, 6]. The next picture in Fig. I is the boeing 737 that lost a top section of the fuselage over Hawaii in 1988 and yet it still managed to land safely. The cause of this fatigue failure was multisite fatigue, which is still only poorly understood. The final picture in Fig. I is a schematic illustration of a microelectronic package to indicate that many of current fracture interests are outside traditional en- gineering. What problems will the future bring? It does not take much imagination to realise that there will be more and more applications of fracture mechanics to tiny functional devices that need structural integrity. Structural problems will not go away, but most will be able to be solved with present techniques provided the problems are remembered. Alan Wells once remarked that the same fracture problem reappears every thirty years, the length of time for a new generation of engineers that have not expe- rienced the problem. In a short paper such as this it is possible to cover only a very small fraction of fracture mechanics The paper will concentrate on quasi-static fracture at ambient temperatures under monotonic loading
‘‘The Engineer’’ that is worth quoting: ‘‘Effects of percussion and frost upon iron... We need hardly say that this is one of the most important subjects that engineers of the present day are called upon to investigate. The lives of many persons, and the property of many more, will be saved if the truth of the matter be discovered––lost if it be not’’ [4]. Apart from the pioneering work, the understanding of fracture waited until the last century of the second Millennium. Even then it took until well into the second half of this century until a rational approach was used by engineers and taught in engineering schools. The author vividly remembers being taught the five theories of strength as an undergraduate in the early 1950s without any real explanation. Amajor spur to the development of fracture theory from 1940 onwards was the brittle fracture of large welded structures such as bridges, ships and oil storage containers. This period is illustrated in Fig. 1 by a photograph of the Schenectady that broke in two while lying in the fitting-out dock in January 1943. One of the most successful applications of linear elastic fracture mechanics (LEFM) is to the reliability assessment of aircraft. Apicture of the ill-fated Comet is shown in Fig. 1. Afast fracture initiated from a fatigue crack that grew from an astro-navigation window and caused the loss of two aircraft in 1953 and 1954. One of the first applications of LEFM was an explanation of this fast fracture [5,6]. The next picture in Fig. 1 is the Boeing 737 that lost a top section of the fuselage over Hawaii in 1988 and yet it still managed to land safely. The cause of this fatigue failure was multisite fatigue, which is still only poorly understood. The final picture in Fig. 1 is a schematic illustration of a microelectronic package to indicate that many of current fracture interests are outside traditional engineering. What problems will the future bring? It does not take much imagination to realise that there will be more and more applications of fracture mechanics to tiny functional devices that need structural integrity. Structural problems will not go away, but most will be able to be solved with present techniques provided the problems are remembered. Alan Wells once remarked that the same fracture problem reappears every thirty years, the length of time for a new generation of engineers that have not experienced the problem. In a short paper such as this it is possible to cover only a very small fraction of fracture mechanics. The paper will concentrate on quasi-static fracture at ambient temperatures under monotonic loading. Fig. 1. The fracture perspective. 534 B. Cotterell / Engineering Fracture Mechanics 69 (2002) 533–553
B. Cotterell Engineering Fracture Mechanics 69(2002 )533-553 Unfortunately only a small fraction of those who have made major contributions to fracture can be mentioned. In Fig. 1, the names stop about thirty years ago since it becomes more difficult to know which ames to omit More names will be mentioned in the following sections, but space will prevent many others who have made important contributions. The author has tried to select those works that have had the most influence but naturally the selection is personal. The author apologises to those whose names are omitted 2. Development of fracture theory up until the Second world war Until Rossmanith [7] rediscovered the work of Weighardt [8]. the first known works yere the two seminal papers of Griffith[9, 10], which form the foundation of modern fracture theory. Griffith was motivated by the need to understand the effect of scratches on fatigue. It was originally thought that it should be possible to estimate the fatigue limit of a scratched component by using either the maximum principal stress criterion, favoured by Lame and Rankine, or the maximum principal strain criterion, fa voured by Ponclet and Saint-Venant. Griffith showed, using the results of Inglis [ll], that scratches could increase the stress and strain level by a factor of between two and six. However, Griffith noted that the maximum stress or strain would be the same on a shaft l in in diameter whether the scratches were one ten- thousandth or one-hundredth of an inch deep provided that they were geometrically similar. These con- clusions were in conflict with the fatigue results and led Griffith to reject the commonly held criteria of rupture. Wieghardt [8] had earlier rejected these strength criteria for a different reason. He was concerned with the paradox that the stresses at the tip of a sharp crack in an elastic body are infinite no matter how small is the applied stress. This fact led him to argue that rupture does not occur when the stress at a point exceeds some critical value, but only when the stress over a small portion of the body exceeds a critical value. The concept of a critical stress intensity factor only just slipped through Wieghardt's fingers. Taylor [12] in either of his classic papers. A possible reason was Griffith's obvious concern that near the tip of a shap states that Griffith was also aware of the paradox. However, Griffith [9, 10] does not mention this paradox crack the small strain assumption is violated and he was hesitant to discuss the stresses at the tip of a sharp crack Reasoning that a simple critical stress or strain criterion could not be used to predict fracture Griffith urned to energy concepts. He realised that a certain minimum work was necessary to produce a fracture, which for an ideal elastic material was the surface free energy. Such a system is conservative and he saw the fracture problem as just an extension of the elastic theory of minimum potential energy. All that had to be done was to consider the potential surface energy as well as the other potential energies of the system. Griffith,s global treatment of the energy balance for a cracked body was praised by Taylor [12] as'the first real advance in understanding the strength of materials. The practical importance of Griffith's work lies in his realisation that the critical stress depends on a length scale the crack length Griffith performed his experiments on a model material glass. From his experiments [9], he estimated the theoretical strength of glass to be about 2 GPa. The observed tensile strength of glass was 170 MPa. Hence Griffith predicted there were flaws of the order of 5 um. Griffith believed that the weakness of glass was due to internal flaws; and indeed believed that the surface layers might be of superior strength because flaws would be oriented parallel to the surface [9). In his 1924 paper that Griffith [10] clearly stated that the weakness(in pure silica) is due almost entirely to minute cracks in the surface, caused by various abrasive actions to which the material has been accidentally subjected after manufacture. Griffith's evidence was that if a strong silica rod was rubbed lightly with any other solid, it immediately lost its great strength However, he did not state that the weakness in glass was due to surface flaws. During the 1920s Joffe [13] and others assumed that it was surface flaws that were responsible for the weakness in glass. Joffe [14]
Unfortunately only a small fraction of those who have made major contributions to fracture can be mentioned. In Fig. 1, the names stop about thirty years ago since it becomes more difficult to know which names to omit. More names will be mentioned in the following sections, but space will prevent many others who have made important contributions. The author has tried to select those works that have had the most influence, but naturally the selection is personal. The author apologises to those whose names are omitted. 2. Development of fracture theory up until the Second World War Until Rossmanith [7] rediscovered the work of Weighardt [8], the first known works devoted to fracture were the two seminal papers of Griffith [9,10], which form the foundation of modern fracture theory. Griffith was motivated by the need to understand the effect of scratches on fatigue. It was originally thought that it should be possible to estimate the fatigue limit of a scratched component by using either the maximum principal stress criterion, favoured by Lame and Rankine, or the maximum principal strain criterion, favoured by Ponclet and Saint-Venant. Griffith showed, using the results of Inglis [11], that scratches could increase the stress and strain level by a factor of between two and six. However, Griffith noted that the maximum stress or strain would be the same on a shaft 1 in. in diameter whether the scratches were one tenthousandth or one-hundredth of an inch deep provided that they were geometrically similar. These conclusions were in conflict with the fatigue results and led Griffith to reject the commonly held criteria of rupture. Wieghardt [8] had earlier rejected these strength criteria for a different reason. He was concerned with the paradox that the stresses at the tip of a sharp crack in an elastic body are infinite no matter how small is the applied stress. This fact led him to argue that rupture does not occur when the stress at a point exceeds some critical value, but only when the stress over a small portion of the body exceeds a critical value. The concept of a critical stress intensity factor only just slipped through Wieghardt’s fingers. Taylor [12] states that Griffith was also aware of the paradox. However, Griffith [9,10] does not mention this paradox in either of his classic papers. Apossible reason was Griffith’s obvious concern that near the tip of a sharp crack the small strain assumption is violated and he was hesitant to discuss the stresses at the tip of a sharp crack. Reasoning that a simple critical stress or strain criterion could not be used to predict fracture Griffith turned to energy concepts. He realised that a certain minimum work was necessary to produce a fracture, which for an ideal elastic material was the surface free energy. Such a system is conservative and he saw the fracture problem as just an extension of the elastic theory of minimum potential energy. All that had to be done was to consider the potential surface energy as well as the other potential energies of the system. Griffith’s global treatment of the energy balance for a cracked body was praised by Taylor [12] as ‘the first real advance in understanding the strength of materials’. The practical importance of Griffith’s work lies in his realisation that the critical stress depends on a length scale, the crack length. Griffith performed his experiments on a model material glass. From his experiments [9], he estimated the theoretical strength of glass to be about 2 GPa. The observed tensile strength of glass was 170 MPa. Hence Griffith predicted there were flaws of the order of 5 lm. Griffith believed that the weakness of glass was due to internal flaws; and indeed believed that the surface layers might be of superior strength because flaws would be oriented parallel to the surface [9]. In his 1924 paper that Griffith [10] clearly stated that the ‘weakness (in pure silica) is due almost entirely to minute cracks in the surface, caused by various abrasive actions to which the material has been accidentally subjected after manufacture’. Griffith’s evidence was that if a strong silica rod was rubbed lightly with any other solid, it immediately lost its great strength. However, he did not state that the weakness in glass was due to surface flaws. During the 1920s Joffe [13] and others assumed that it was surface flaws that were responsible for the weakness in glass. Joffe [14] B. Cotterell / Engineering Fracture Mechanics 69 (2002) 533–553 535
B. Cotterell Engineering Fracture Mechanics 69(2002)533-553 presented his work inferring that the strength of rock salt was due to surface flaws because when the surface layer was dissolved in warm water the strength increased, at the same Delft conference as griffith From the 1920s onwards there was a search for flaws in glass. The separation across a surface flaw in glass is of the order of 50 nm, only about one-tenth of the wavelength of light, and undetectable optically. It was not until 1933 that the experiments on mica by Orowan [15] proved conclusively that the reduction in strength was due to flaws. The usual tensile strength of mica is between 200-300 MPa, but Orowan ob- tained strengths of more than 3 GPa by stressing only the central strip of a sheet of mica using grips that were much narrower than the sheet. The small value of the usual tensile strength of mica is due to the presence of cracks at the edge of the sheet. The cleavage plane is near perfect. The first direct evidence for the existence of surface flaws in glass came by chance in 1935 during experiments by Andrade and Mar tindale [16] on the properties of thin films of metal, followed later a series of experiments on various glasses using sodium from a vapour to"decorate"the surface cracks [17] The experiments demonstrating the reversibility of fracture of obreimoff [18]in 1930 deserve a mention The fracture of a Griffith crack is unstable so there is no possibility of reversibility. However, Obreimoff studied the fracture of mica using a stable geometry. Mica has a very pronounced cleavage plane and al- most atomically perfect surfaces can be produced by cleavage. Obreimoff used a glass wedge to cleave thin lamellar of mica 0. 1-0.2 mm thick from a block of mica. The fracture of such cantilever specimens under fixed deflection conditions is stable and can be analysed with the engineers' theory of bending and is the start of a love affair between fracture mechanists and the double cantilever beam specimen. Since Obreimoff used a stable geometry, he found that a crack could grow under the combined effect of mechanical energy and moisture in the air. Obreimoff demonstrated the reversibility of fracture, the two mica surfaces re- adhering when the wedge was redrawn. Under atmospheric pressure the equilibrium measured surface energy of a healed crack was slightly less than for a virgin crack 3. The Second World War and the brittle fracture problem The transition temperature from ductile to brittle behaviour in structural steel of the time was around 20C. Problems had arisen from the brittle behaviour of steel with the introduction of high volume steel production by the Bessemer process in the 1860s. These problems were recognised by some engineers such as David Kirkaldy. However in riveted structures brittle fractures rarely caused catastrophes because a fracture was usually arrested at the edge of the plate in which it initiated. The earliest recorded case of brittle fracture in steel is that of a 75 m high by 5 m diameter, water standpipe at Gravesend, Long Island, NY in 1898[19] a generation after the prophetic leader in"The Ensg yd wol. 1. Without proper fracture control, welding introduces the elements necessary for brittle fracture in steel: high residual stresses equal to the yield strength, a heat affected zone adjacent to a weld with a much higher transition temperature than the parent plate, and crack-like defects. Since a welded structure is continuous, unstable brittle fracture can easily run thorough a major part of its section and cause a catastrophe. The first brittle fracture in a large welded structure occurred just before World War II in the Vierendeel Truss Bridge in Hasselt, Belgium followed by failures in similar Belgium bridges during the war [19]. However, the Allies knew little about these fractures during the war and what started the investigation of brittle fracture in earnest were the widespread fractures in the welded Liberty Ships. There were 145 structural failures in Liberty hips where the vessel was either lost or the hull so weakened to be dangerous, a further 694 ships suffered major fractures requiring immediate repair [20]. A consequence of these failures was the setting up of the US Navy Ship Structure Committee and the Admiralty Ship Welding Committee. The British Committee was under the chairmanship of John Baker who assigned the metallurgical investigation to Constance Ti
presented his work inferring that the strength of rock salt was due to surface flaws because when the surface layer was dissolved in warm water the strength increased, at the same Delft conference as Griffith. From the 1920s onwards there was a search for flaws in glass. The separation across a surface flaw in glass is of the order of 50 nm, only about one-tenth of the wavelength of light, and undetectable optically. It was not until 1933 that the experiments on mica by Orowan [15] proved conclusively that the reduction in strength was due to flaws. The usual tensile strength of mica is between 200–300 MPa, but Orowan obtained strengths of more than 3 GPa by stressing only the central strip of a sheet of mica using grips that were much narrower than the sheet. The small value of the usual tensile strength of mica is due to the presence of cracks at the edge of the sheet. The cleavage plane is near perfect. The first direct evidence for the existence of surface flaws in glass came by chance in 1935 during experiments by Andrade and Martindale [16] on the properties of thin films of metal, followed later a series of experiments on various glasses using sodium from a vapour to ‘‘decorate’’ the surface cracks [17]. The experiments demonstrating the reversibility of fracture of Obreimoff [18] in 1930 deserve a mention. The fracture of a Griffith crack is unstable so there is no possibility of reversibility. However, Obreimoff studied the fracture of mica using a stable geometry. Mica has a very pronounced cleavage plane and almost atomically perfect surfaces can be produced by cleavage. Obreimoff used a glass wedge to cleave thin lamellar of mica 0.1–0.2 mm thick from a block of mica. The fracture of such cantilever specimens under fixed deflection conditions is stable and can be analysed with the engineers’ theory of bending and is the start of a love affair between fracture mechanists and the double cantilever beam specimen. Since Obreimoff used a stable geometry, he found that a crack could grow under the combined effect of mechanical energy and moisture in the air. Obreimoff demonstrated the reversibility of fracture, the two mica surfaces readhering when the wedge was redrawn. Under atmospheric pressure the equilibrium measured surface energy of a healed crack was slightly less than for a virgin crack. 3. The Second World War and the brittle fracture problem The transition temperature from ductile to brittle behaviour in structural steel of the time was around 20 C. Problems had arisen from the brittle behaviour of steel with the introduction of high volume steel production by the Bessemer process in the 1860s. These problems were recognised by some engineers such as David Kirkaldy. However in riveted structures brittle fractures rarely caused catastrophes because a fracture was usually arrested at the edge of the plate in which it initiated. The earliest recorded case of brittle fracture in steel is that of a 75 m high by 5 m diameter, water standpipe at Gravesend, Long Island, NY in 1898 [19] a generation after the prophetic leader in ‘‘The Engineer’’ [4]. Electric arc welding for the construction of large steel structures was just introduced prior to World War II. Without proper fracture control, welding introduces the elements necessary for brittle fracture in steel: high residual stresses equal to the yield strength, a heat affected zone adjacent to a weld with a much higher transition temperature than the parent plate, and crack-like defects. Since a welded structure is continuous, unstable brittle fracture can easily run thorough a major part of its section and cause a catastrophe. The first brittle fracture in a large welded structure occurred just before World War II in the Vierendeel Truss Bridge in Hasselt, Belgium followed by failures in similar Belgium bridges during the war [19]. However, the Allies knew little about these fractures during the war and what started the investigation of brittle fracture in earnest were the widespread fractures in the welded Liberty Ships. There were 145 structural failures in Liberty ships where the vessel was either lost or the hull so weakened to be dangerous, a further 694 ships suffered major fractures requiring immediate repair [20]. Aconsequence of these failures was the setting up of the US Navy Ship Structure Committee and the Admiralty Ship Welding Committee. The British Committee was under the chairmanship of John Baker who assigned the metallurgical investigation to Constance Tipper. 536 B. Cotterell / Engineering Fracture Mechanics 69 (2002) 533–553
B. Cotterell Engineering Fracture Mechanics 69(2002 )533-553 Brittle fracture was initially seen as an almost purely metallurgical problem, I if Griffith's work was considered at all it was simply to point out the importance of a notch. The main aim up until the 1960s was to determine the transition temperature at which the fracture behaviour changed from ductile to cleavage As early as 1909, Ludwik [21] explained the phenomenology of the transition from ductile to cleavage behaviour. He suggested that the cohesive strength was little affected by temperature, but there was a marked increase in the yield strength of low carbon steel as the temperature decreased so that a particular mperature cleavage fracture became easier than yielding. The effect of a notch on the transition tem- perature was seen to be primarily due to a constraint on yielding and Orowan [22], using Ludwik's concept, showed how a notch would increase the transition temperature. The Charpy test [23], originally introduced to deal with the problem of temper brittleness, was one of the original and the most lasting of the small- scale notch bend tests to assess the transition temperature in steel The realisation that there was a size effect in the brittle fracture of steel took sometime to develop probably due to the limited size range of laboratory specimens, and as late as 1960, Biggs [20]could write: "The fun damental problems associated with size effect have received limited attention. Size effect was recognised as early as 1932 by Docherty [24] but only slowly became widely appreciated. At the Navy Research Labora tories in Washington, Irwin [25] and Shearin, Ruark and Trimble [26] at the University of Carolina were amining size effect in the late 1940s. At the University of Illinois, wilson, Hetchtman and bruckner were using their huge 3,000,000 lb hydraulic testing machine to test plates 3/4 in. thick up to 72 in. wide[27] c It was Wells [28] who developed the first fracture test that fully simulated a welded plate structure. He igned a special simple 600-ton testing machine that was capable of testing I in. thick 36 x 36 in. butt elded plates [29]; later models had capacities up to 4000 tons [30]. The Wells wide plate test consists of a butt-welded plate that has a fine saw cut made into the weld preparation. This saw cut is not fully buried by e weld. Since the plate is wide, full residual stresses can develop that are similar to those that would occur in normal construction. The welded plates could be tested either as-welded or after heat treatment and were welded into the test rig. The plates were cooled with dry-ice to the desired temperature before testing. The first results using the wide plate test were published in 1956[28]. Typical results for low carbon steel are shown in Fig. 2 [31]. Those plates welded with rutile electrodes tended to have precracks at the saw cut Fractures were often initiated in precracked specimens at low stress, or occurred spontaneously on cooling, these arrested at the edge of the tensile residual stress zone along the weld. The transition from high stress fractures at the yield strength to low stress fractures did not always occur at the Charpy transition temperature. The Wells wide plate test was widely accepted and various versions of the test were adopted around the world Though there are certainly size effects in the brittle fracture of steel, it is true that the problem was largely a metallurgical one. It was solved metallurgically by developing steels with lower transition temperatures However, later metallurgical improvements to structural steel to increase their strength, brought problems with fast ductile fracture in welded oil and gas pipe lines. For comparatively thin walled pipe lines, th problem too in its turn was solved metallurgically, but in heavy sections such as found in offshore oil rigs or nuclear power plants, ductile fracture has to kept at bay by elasto-plastic fracture mechanics(EPFM) 4. The development of linear elastic fracture mechanics Until the late 1940s, Griffiths pioneering work [9, 10] was not seen as having very much relevance to engineering. Griffith chose glass, one of the most brittle materials, for his model material. The size effect in This belief was carried into the 1960s by some metallurgists. In the discussion of a paper by the author at an Australian conference in the 1960s, the main criticism from one metallurgist did not concern the substance of the paper, but that its author was not a tallurgist
Brittle fracture was initially seen as an almost purely metallurgical problem, 1 if Griffith’s work was considered at all it was simply to point out the importance of a notch. The main aim up until the 1960s was to determine the transition temperature at which the fracture behaviour changed from ductile to cleavage. As early as 1909, Ludwik [21] explained the phenomenology of the transition from ductile to cleavage behaviour. He suggested that the cohesive strength was little affected by temperature, but there was a marked increase in the yield strength of low carbon steel as the temperature decreased so that a particular temperature cleavage fracture became easier than yielding. The effect of a notch on the transition temperature was seen to be primarily due to a constraint on yielding and Orowan [22], using Ludwik’s concept, showed how a notch would increase the transition temperature. The Charpy test [23], originally introduced to deal with the problem of temper brittleness, was one of the original and the most lasting of the smallscale notch bend tests to assess the transition temperature in steel. The realisation that there was a size effect in the brittle fracture of steel took sometime to develop probably due to the limited size range of laboratory specimens, and as late as 1960, Biggs [20] could write: ‘‘The fundamental problems associated with size effect have received limited attention.’’ Size effect was recognised as early as 1932 by Docherty [24] but only slowly became widely appreciated. At the Navy Research Laboratories in Washington, Irwin [25] and Shearin, Ruark and Trimble [26] at the University of Carolina were examining size effect in the late 1940s. At the University of Illinois, Wilson, Hetchtman and Bruckner were using their huge 3,000,000 lb hydraulic testing machine to test plates 3=4 in. thick up to 72 in. wide [27]. It was Wells [28] who developed the first fracture test that fully simulated a welded plate structure. He designed a special simple 600-ton testing machine that was capable of testing 1 in. thick 36 36 in. buttwelded plates [29]; later models had capacities up to 4000 tons [30]. The Wells wide plate test consists of a butt-welded plate that has a fine saw cut made into the weld preparation. This saw cut is not fully buried by the weld. Since the plate is wide, full residual stresses can develop that are similar to those that would occur in normal construction. The welded plates could be tested either as-welded or after heat treatment and were welded into the test rig. The plates were cooled with dry-ice to the desired temperature before testing. The first results using the wide plate test were published in 1956 [28]. Typical results for low carbon steel are shown in Fig. 2 [31]. Those plates welded with rutile electrodes tended to have precracks at the saw cut. Fractures were often initiated in precracked specimens at low stress, or occurred spontaneously on cooling, these arrested at the edge of the tensile residual stress zone along the weld. The transition from high stress fractures at the yield strength to low stress fractures did not always occur at the Charpy transition temperature. The Wells wide plate test was widely accepted and various versions of the test were adopted around the world. Though there are certainly size effects in the brittle fracture of steel, it is true that the problem was largely a metallurgical one. It was solved metallurgically by developing steels with lower transition temperatures. However, later metallurgical improvements to structural steel to increase their strength, brought problems with fast ductile fracture in welded oil and gas pipe lines. For comparatively thin walled pipe lines, this problem too in its turn was solved metallurgically, but in heavy sections such as found in offshore oil rigs or nuclear power plants, ductile fracture has to kept at bay by elasto-plastic fracture mechanics (EPFM). 4. The development of linear elastic fracture mechanics Until the late 1940s, Griffith’s pioneering work [9,10] was not seen as having very much relevance to engineering. Griffith chose glass, one of the most brittle materials, for his model material. The size effect in 1 This belief was carried into the 1960s by some metallurgists. In the discussion of a paper by the author at an Australian conference in the 1960s, the main criticism from one metallurgist did not concern the substance of the paper, but that its author was not a metallurgist! B. Cotterell / Engineering Fracture Mechanics 69 (2002) 533–553 537